The molecular pathogenesis of fully virulent, wild-type Y. pestis in relevant animal models has been relatively neglected because of the scarcity of secure BSL-3 facilities and trained personnel certified to work with this Class A select agent. The threat of bioterrorism and the emergence of multiply-antibiotic resistant strains of Y. pestis increases the urgency for a more detailed understanding of the host-pathogen relationship at the molecular level that may lead to the design of improved medical countermeasures and diagnostics. We have established mouse and rat models of bubonic plague that incorporate flea-to-rodent transmission to investigate the role of specific Y. pestis virulence factors and to characterize the host response to naturally acquired infection. We have characterized the kinetics, microbiology, and histopathology of bubonic plague in rats following intradermal injection of Y. pestis, and used this model to characterize the gene expression profile of Yersinia pestis in the infected lymph node during bubonic plague using whole-genome microarray technology. Based on these results, we tested the virulence of specific Y. pestis mutant strains to determine the role of bacterial genes predicted to be important in resistance to the host innate immune response. During the past year we have completed a characterization of the rat gene expression response to infection in the lymph node during bubonic plague, which showed that Y. pestis actively inhibits the innate immune response until late in infection. We have developed models, including in vivo imaging techniques, to examine host-parasite interactions in the dermis after transmission by flea bite and the effects of flea saliva on this interaction. In collaboration with Dr. Susan Buchanan (NIDDK), we continued to study the role of Y. pestis outer surface proteins that are upregulated during infection. In the 2009 fiscal year, we have shown that one of them is required for virulence and conducted initial trials to test its efficacy for use in an anti-plague vaccine and as a serological marker for plague diagnosis. Also in collaboration with Dr. Buchanan, we have begun to evaluate the efficacy of a novel hybrid lysin protein as a therapeutic agent for plague.